NAME PER DATE DUE ACTIVE LEARNING IN CHEMISTRY EDUCATION "ALICE" CHAPTER 21 ACIDS AND BASES. Behavior In Water , A.J.

Transcription

1 NAME PER DATE DUE ACTIVE LEARNING IN CEMISTRY EDUCATIN "ALICE" CAPTER 21 ACIDS AND BASES Behavior In Water , A.J. Girondi

2 NTICE F RIGTS All rights reserved. No part of this document may be reproduced or transmitted in any form by any means, electronic, mechanical, photocopying, or otherwise, without the prior written permission of the author. Copies of this document may be made free of charge for use in public or nonprofit private educational institutions provided that permission is obtained from the author. Please indicate the name and address of the institution where use is anticipated A.J. Girondi, Ph.D. 505 Latshmere Drive arrisburg, PA alicechem@geocities.com Website: , A.J. Girondi

3 SECTIN 21.1 Acids and Bases - The Arrhenius Definitions In preceding chapters you have studied substances such as gases, salts, metals, and so forth. These labels describe classes of chemical substances which have certain properties in common. This chapter will introduce you to two additional classes of substances known as acids and bases. Many years ago it was found that when certain substances were placed into water, the resulting solutions had a sour taste. When the early alchemists discovered this sour taste, they called these substances acids. The alchemists were fond of using Latin names and phrases when describing their work. The Latin word meaning "sour" is acidus. The alchemists discovered that these acid solutions were sometimes capable of dissolving compounds that would not dissolve in plain water. You are probably familiar with the sour taste of vinegar. Vinegar contains acetic acid. The Latin word for vinegar is acetum, reflecting its sour taste. Alchemists also worked with another kind of substance prepared from the ashes of dried plants. When these ashes were placed in water, some of the components in the ashes dissolved in the water. It was found that when these "ash solutions" were mixed with acid solutions, the sour taste of the acid disappeared. These substances derived from ashes were named bases. Early chemists found that a base could neutralize, or cancel out, the properties of an acid. Chemical descriptions of acids and bases have developed and improved during the past 300 years. The actual ions and molecules present in acids and bases have been identified. It was found that the common ion present in acid solutions was the positively charged hydrogen ion, 1+, while the most common ion found in solutions of bases was the negatively-charged 1- ion. What is the name of this ion? {1} In modern chemistry there are three ways to define acids and bases. In this chapter, we will examine two of these ways. Please keep in mind that in this chapter we will be discussing how acids behave when you put them into pure water, and how bases behave when you put them into pure water. In the next chapter, you will study how acids and bases react with each other. The first and simplest definition of acids was provided by the Swedish chemist, Arrhenius, in e defined an acid as a compound that contains hydrogen and which would produce hydrogen ions ( 1+ ) when you dissolve it in water. For example, when hydrogen chloride gas (Cl) is dissolved in water it breaks up or "dissociates" into hydrogen ions and chloride ions: Cl(g) -----> 1+ (aq) + Cl 1- (aq) This solution which contains hydrogen and chloride ions is called hydrochloric acid. It is the acid which aids digestion in your stomach. Its acidic properties are due to the presence of the hydrogen ions, 1+. We will generally show hydrogen as the first element in the formula of an acid. In some acids, hydrogens which are part of a polyatomic ion do not dissociate into hydrogen ions. Cl C acetate ion In the case of acetic acid (the acid in vinegar) the first in the formula represents what we call an "acidic" hydrogen, which is an that can form an 1+ ion when the molecule is put into water. The three 's which are part of the acetate ion (C ) are not "acidic" 's: C232(aq) <===> 1+ (aq) + C (aq) , A.J. Girondi

4 The Arrhenius definition of bases is the simplest one. According to Arrhenius, a base is a compound that contains the hydroxide ion, and produces hydroxide ions ( 1- ) when it is dissolved in water. When solid sodium hydroxide is dissolved in water, it breaks up or "dissociates" into sodium ions and hydroxide ions: Na(s) -----> Na 1+ (aq) + 1- (aq) A water solution of Na has the properties of a base because of the presence of the hydroxide ions. Sometimes basic solutions are described as being "alkaline." The Arrhenius Definitions: An acid produces 1+ in water solution A base produces 1- in water solution General Properties of Acidic Solutions they taste sour they neutralize bases they affect chemical indicators they are electrolytes, meaning they conduct electricity Problem 1. Complete the following equations, showing how the acids listed dissociate in water to form hydrogen ions and a negatively-charged ion (anion). a. Br(aq) b. I(aq) c. Cl4(aq) General Properties of Basic Solutions they taste bitter they neutralize acids they affect chemical indicators they feel slippery they are electrolytes, meaning they conduct electricity Problem 2. Complete the following equations, showing how the bases listed dissociate in water to form 1- ions. a. K(s) b. Li(s) c. Cs(s) , A.J. Girondi

5 SECTIN 21.2 The Bronsted Lowry Definitions Arrhenius's definitions were generally accepted by chemists of his time. They explained many unanswered questions about acids and bases. is definitions of acids and bases are still widely used by chemists who work with aqueous (water) solutions. owever, eventually compounds were discovered that had the properties of bases, but which did not contain the 1- ion. This meant that a better definition of bases was needed. The Arrhenius definition was no longer adequate. A definition was needed which could explain why some compounds, other than those containing hydroxide, had "basic" properties. The term "alkaline" is sometimes used to describe substances which have "basic" properties. To solve this problem, a second definition of acids and bases was suggested by Thomas M. Lowry and Johannes N. Bronsted. It will be helpful for you to realize that a hydrogen ion ( 1+ ) can also be called a proton, since all that is left of a hydrogen atom which has lost its electron is a proton. ydrogen Ion = 1+ = a proton 1+ A common hydrogen atom consists of one proton in the nucleus and one electron. If a hydrogen atom loses its electron, it becomes a hydrogen ion - just a proton. Bronsted and Lowry proposed that an acid be defined as a molecule or ion that can give away or donate a proton ( 1+ ) to some other particle. A base was defined as a substance that can combine with or accept a proton ( 1+ ) from some other particle. According to the Bronsted Lowry concept, an acid became known as a proton donor, and a base as a proton acceptor. The Bronsted Lowry Definitions An acid donates a proton ( 1+ ) A base accepts a proton ( 1+ ) Note that the Arrhenius definition of an acid and the Bronsted Lowry definition of an acid are very similar. Both definitions refer to the formation of 1+ : Cl ----> 1+ + Cl 1-. owever, the Arrhenius definition of a base refers only to substances which can provide 1- ions in solution, whereas, the Bronsted Lowry definition of a base refers to any particle which can accept a hydrogen ion (proton), 1+. Perhaps the following example will help. Arrhenius would consider Na to be a base because it forms 1- when you put it into water solution: Na(s) ----> Na 1+ (aq) + 1- (aq) Bronsted and Lowry would consider Na to be a base because a solution of it contains the 1- ion which is a proton acceptor: 1+ (aq) + 1- (aq) ----> (l) When 1- accepts a proton its forms a molecule of water,. Now remember, when Arrhenius defined a base he was thinking only of 1-. owever, other particles such as the fluoride ion, F 1-, can also act a proton acceptors. For example: 1+ (aq) + F 1- (aq) ----> F(aq) , A.J. Girondi

6 Solutions which contain the fluoride ion have "basic" properties similar to solutions which contain the hydroxide ion, 1-. ther particles which can function as bases (proton-acceptors) of this kind include both molecules and ions like C232 1-, N3, C3 2-, and many others. Therefore, many more particles can be classified as bases according to Bronsted Lowry than according to Arrhenius. ere are a few more equations which illustrate how these bases function as proton-acceptors: 1+ (aq) + C (aq) ----> C232(aq) 1+ (aq) + N3(aq) ----> N4 1+ (aq) 1+ (aq) + C3 2- (aq) ----> C3 1- (aq) What is it about particles like 1-, F 1-, C232 1-, N3, and C3 2- that allows them to function as proton acceptors? If you look at the electron dot structure of these ions, you will see that they have one or more unshared pairs of electrons: The 1- ion has 3 unshared pairs of electrons, while the F 1- ion has 4 unshared pairs. 1- F hydroxide ion (a base) 1- fluoride ion (a base) The 1+ ion is seeking the stable helium configuration (1s 2 ). It can achieve that configuration by sharing a pair of electrons with another particle. So, protons ( 1+ ) tend to bond to particles which have an unshared pair of electrons in their valence shells. Many such particles can act as proton acceptors which are bases according to the {2} definition. Check out the electron-dot structures of 1- and the F 1- ions below: F 1- F If you examine the electronic structures of other bases like N3 molecules or acetate ions, C232 1-, you will see that they also have unshared pairs of electrons: N N C C C C You might ask, "Why do acids lose 1+ when they are put into water?" Well, the reason is that water molecules take them! You see, since water has two unshared pairs of electrons on the oxygen atom, it too can accept protons and function as a base. Note the electron-dot structure of water shown at right , A.J. Girondi

7 To illustrate how water can act as a proton-acceptor, consider the equation below showing the reaction or "dissociation" of Cl in water: Cl(g) + 2(l) ----> 3 1+ (aq) + Cl 1- (aq) The shorthand way of writing this equation is: Cl(g) -----> 1+ (aq) + Cl 1- (aq) (Note that Cl is a gas and therefore is accompanied by the (g) subscript before it is put into water. Most of the other acids you will see will be written using the (aq) subscript.) When you include water in the equation, you must represent the hydrogen ion as 3 1+ instead of as 1+. This is actually a more accurate representation of what happens. The 3 1+ particle is known as the hydronium ion. It is a water molecule which is bonded to a proton ( 1+). Its electron-dot structure is shown at right. The hydronium ion forms when water acts as a base and accepts a proton from an acid molecule like Cl. 1+ Cl + Equations representing what happens when you put an acid molecule in water can, therefore, be written in two ways. You can choose to show the water and the hydronium ion (the more accurate way to do it), or you can choose not to show them (the shorthand way of writing it). Below are the equations representing what happens when you put nitric acid in water: N3(aq) + 2(l) ----> 3 1+ (aq) + N3 1- (aq) or 1+ N3(aq) ----> 1+ (aq) + N3 1- (aq) + Cl 1-1+ According to the Bronsted-Lowry definition, water can act as a base since it can accept protons. (Later you will learn that water can also give away a proton and, therefore, act as an acid.) Problem 3. For each of the acids below, write two forms of the equation which represents what happens when you put them into water. a. Br b. Cl4 Not all particles with unshared pairs of electrons make good bases (proton acceptors) in water. For example, the chloride ion has the necessary unshared pairs of electrons, but it is not a basic ion. It is described as being neutral. It is NT a proton acceptor: Cl no reaction Some other neutral ions include Br 1-, I 1-, N3 1-, S4 2-, and Cl , A.J. Girondi

8 SECTIN 21.3 Strong Acids Versus Weak Acids A. Strong Acids The reason that solutions of acids and bases are electrolytes is because they contain ions. The ions move about and carry the electric charge through the solution. The equation below demonstrates the reaction of hydrochloric acid in water. Note that the ions produced are "aqueous" meaning in water solution. Acids are hydrogen compounds that form water solutions which contain ions, one of which is the hydrogen ion. For example: I(aq) ----> 1+ (aq) + I 1- (aq) or I(aq) + 2(aq) ----> 3 1+ (aq) + I 1- (aq) There are only six strong acids, but there are many, many weak acids. The six strong acids are Br, Cl, I, N3, 2S4, Cl4. You should memorize the six strong acids. You may be wondering what makes an acid strong or weak. Acids such as Cl and N3, are strong acids because they dissociate completely to form ions when they are put into water. In other words, all of the molecules of a strong acid will dissociate into ions when you put the acid into water solution. We say that they are "100 percent dissociated in water." Solutions of strong acids, therefore, contain a high concentration of hydrogen ions. Perchloric acid, Cl4, is an example: Cl4(aq) ----> 1+ (aq) + Cl4 1- (aq) R Cl4(aq) + 2(l) ----> 3 1+ (aq) + Cl4 1- (aq) Cl hydrochloric acid I hydroiodic acid Br hydrobromic acid TE SIX STRNG ACIDS 2S4 sulfuric acid N3 nitric acid Cl4 perchloric acid B. Weak Acids Acetic acid, C232, is classified as being weak. Because of the nature of the bonding between the acidic hydrogen and the acetate ion, a molecule of acetic acid does not dissociate very much in water. As a result, most molecules of weak acids remain in the form of molecules when they are put into water. Many of the ions which form when the weak acid molecules dissociate will recombine to form the original molecules. Thus, the concentration of hydrogen ions is lower than it would have been if the acid had been strong. nly a small percentage of molecules of a weak acid will be dissociated at any given point in time. An equilibrium is established in which the equilibrium is strongly favored toward the reactants (<---): , A.J. Girondi

9 C232(aq) + 2(l) <====> 3 1+ (aq) + C (aq) double arrow indicates equilibrium r, using the shortcut (but less accurate) representation: low concentration of hydronium ions C232(aq) <====> 1+ (aq) + C (aq) The use of the double-headed arrow indicates that the acid is weak and exists mostly in the form of molecules (C232) rather than as ions. Equilibrium is characteristic of {3} acids in water. When equilibrium is established, the system contains mostly reactants and, therefore, not many ions. This fact provides us with a method for determining the strength of acids. The greater the concentration of ions in an acid solution, the better the solution will conduct electricity. Since stronger acids dissociate into ions much, much better than weak ones do, the stronger acids are {4} conductors of electricity. You will use this property to determine the relative strengths of some acids in the next activity. There are many weak acids. owever, at this point you will be expected to memorize the names and formulas of only the three listed below. SME WEAK ACIDS C232 Acetic Acid F ydrofluoric Acid 3P4 Phosphoric Acid Weak acids form an equilibrium system in water in which the reverse reaction is favored. Why? Each of the two reactions involved in such an equilibrium system is an acid-base reaction. In the system shown below, F is a weak acid. In the forward reaction (---->) the acid is F and the base is {5}. That is, the F is donating a proton, and the 2 is accepting it to form the products on the right. In the reverse reaction (<----) the acid is 3 1+ and the base is {6}. That is, the 3 1+ is donating a proton, and the F 1- is accepting it to form the products on the left: F + 2 <====> F 1- acid base acid base It turns out that 3 1+ is a stronger proton donor (acid) than F and F 1- is a stronger proton acceptor (base) than 2. Therefore, the reverse reaction (<----) is better than the forward (---->) one. When equilibrium is established, there will be much more F and 2 in the solution than 3 1+ and F 1-. Solutions of weak acids, therefore, have a relatively low concentration of 3 1+ ions. Problem 4. Complete the following equations, showing how the weak acids listed below dissociate in water to form ions. The anions contained in these acids are Cl3 1- and Cl 1-. Be sure to use a doubleheaded arrow in the equation. Include 2 in the equations. a. Cl3(aq) b. Cl(aq) , A.J. Girondi

10 ACTIVITY 21.4 Comparing the Conductivity of Strong and Weak Acids There is a fairly simple method that can be used to determine the extent to which an acid or base dissociates. This method involves measuring the conductivity of a solution. When ions are present in a solution, it is possible for the solution to conduct electricity. ther things being equal, the greater the concentration of ions in a solution, the greater the electrical current that will pass through the solution. The number of ions which form when an acid or base is added to water depends on the degree of dissociation. Since stronger acids and bases dissociate much more than weaker ones do, they are much better conductors of electric current. The greater the electrical conductivity of an acid or base solution, the stronger it is. Your teacher will give you specific instructions about how to use the conductivity device. Use it to measure the conductivity of the three acids listed in Table Record your observations in the spaces provided in the table. If the apparatus has a meter, record the meter reading in the table. If it has a light bulb, record the strengths of the acids as high, medium, or low depending on the brightness of the bulb. If the bulb does not light, this is probably because the ion concentration is too low to allow enough current to flow to light the bulb. It does not necessarily mean that there are no ions in the solution. (int: one of the acids should be rated "high;" one should be rated "medium;" and one should be rated "low." You determine which is which. Table 21.1 Conductivity of Acid Solutions Acid Formula Acid Name Conductivity 0.01M Cl hydrochloric 0.01M C232 acetic 0.01M 8C67 citric Based on the results, what conclusions can you draw about the relative strengths of these three acids? {7} Even though all three acid solutions have the same concentration, they do not conduct the same amount of electric current and are not equally strong. Why not? {8} SECTIN 21.5 The Dissociation Constant of a Weak Acid, K a As you have already learned, weak acids form an equilibrium system when they are put into water. There is both a forward and a reverse reaction. We can write equilibrium expressions for weak acid systems. Let's consider the acetic acid equilibrium system: C232(aq) + 2(l) <====> 3 1+ (aq) + C (aq) You should recall that an equilibrium expression consists of the product of the molar concentrations of the , A.J. Girondi

11 products divided by the product of the molar concentrations of the reactants. Furthermore, you may recall that solids and pure liquids are not included in equilibrium expressions. Water is a pure liquid in the system above. The equilibrium constant for a weak acid is called the dissociation constant and is given the symbol, K a. The Ka expression for the acetic acid system is given below. Compare the Ka expression to the equation shown above. [ K a = 3 1+ ] [C ] [C ] Problem system. 5. The two equilibrium equations below involve weak acids. Write the Ka expression for each a. N2(aq) + 2(l) <===> 3 1+ (aq) + N2 1- (aq) Ka = b. CN(aq) + 2(l) <===> 3 1+ (aq) + CN 1- (aq) Ka = Look at the expressions for Ka which you wrote above. Note that as the concentration of hydronium ions, 3 1+, increases, so does the value of Ka. Weak acids vary considerably in strength. Some are much stronger than others, although none of them approach the strength of the six "official" strong acids. Thus, the weak acids which are "strongest" have larger Ka values. Indeed, by comparing Ka values, you can determine which of any given set of acids is strongest. Three weak acids are listed in Table Which is strongest? {9} Weakest? {10} Table 21.2 Ka Values of Selected Weak Acids Acid Dissociation Constant hypochlorous acid, Cl Ka = 3.2 X 10-8 formic acid, C Ka = 1.8 X 10-4 phosphoric acid, 3P4 Ka = 7.1 X 10-3 Reference sources such as textbooks and The andbook of Chemistry and Physics contain tables of Ka values of many weak acids. SECTIN 21.6 The Behavior of Strong and Weak Bases In Water A. Strong Bases Bases are also divided into groups that are strong and weak. The strong bases are the hydroxide compounds of most of the Group 1A and 2A metals. Examples include Li, Na, K, Ca()2, etc. All other bases are considered to be weak. Strong bases are completely dissociated in water. Strong base: Na(s) > Na 1+ (aq) + 1- (aq) , A.J. Girondi

12 Strong bases form solutions which contain lots of ions. The base (Na in this case) simply breaks apart in water. They are considered to be strong bases because 100% of the compound dissociates into individual ions when it dissolves. The hydroxides of family 1A metals, like Na, are potent bases for two reasons: (1) they are very soluble in water, and (2) they are 100 percent dissociated in solution. As a result, strong bases of family 1A can produce lots of 1- ions in solution. The high concentration of 1- ions in strong bases makes them dangerous. The strong bases are the hydroxide compounds of the solid metals in families1a and 2A of the periodic table. (Except for beryllium and magnesium) Using a periodic table, you should be able to write the formulas of the strong bases from memory. The hydroxide compounds of the family 2A metals, like Ca()2, are considered to be strong because, like family 1A hydroxides, they too are 100% dissociated in solution. owever, the family 2A hydroxides are not nearly as soluble as those of family 1A. As a result, while they are strong bases, they are not as potent because they do not form solutions with high concentrations of 1- (since they are not very soluble). nly a small amount of Ca()2 will dissolve before its solution becomes saturated. In fact, Ca()2 is called "lime" and is mild enough to be used to neutralize acids in lawn and garden soils. Mg()2 is used in some stomach antacids like "milk of magnesia." You wouldn't want to use Na for that purpose! Na is the active ingredient in many drain cleaners! Strong base: Ca()2(s) ----> Ca 2+ (aq) (aq) [Ca()2 is 100% dissociated, but not much will dissolve.] Water is not included in equations which show the dissociation of strong bases. Strong and weak acids actually react with water so 2 can be included in the equation. Strong bases do not react with water; they just come apart (dissociate). Problem 6. Write equations showing the complete dissociation of the following strong bases in water. a. Rb(s) ----> b. Ba()2(s) ----> B. Weak Bases Weak bases are in some ways similar to weak acids. For example, they form ions when you dissolve them in water. Furthermore, they form equilibrium systems in water, because they dissociate only slightly (similar to weak acids). Some are polar covalent molecules which react with water to form ions. The most common example of a weak base is ammonia, N3. Ammonia: N3(g) + (l) <===> N4 1+ (aq) + 1- (aq) weak base weak acid Note: Remember that water can also be written as. We will use this form of the formula in equations dealing with weak bases, because it makes it easier to see what is happening , A.J. Girondi

13 Some weak bases are ions such as the fluoride ion, F 1- : Weak base: F 1- (aq) + (l) <===> F(aq) + 1- (aq) base acid acid base The electron dot notation of the fluoride ion, F 1-, molecule is shown at right. What is the feature of this molecule that allows it to function as a base? {11} F 1- Notice that water acts as an acid in the last two equations. In previous sections, you saw water acting as a base. Water can act as an acid because it contains hydrogen which can form ions ( 1+ ), and it contains oxygen which has unshared pairs of electrons which allow it to act as a Bronsted Lowry base. Substances such as water which can act as either acids or bases are said to be amphoteric. Can act as an acid by giving away hydrogen in the form of 1+ WATER Can act as a base by attracting 1+ to one of its unshared electron pairs. There is an important difference between weak bases and strong bases. Both produce hydroxide ions, 1-, when you dissolve them in water; however, with strong bases the 1- comes directly from the strong base, like Na: Na(s) ----> Na 1+ (aq) + 1- (aq) Note that the formula for a weak base like ammonia, N3, does not contain hydroxide. The most common and important weak bases are not hydroxide compounds. Yet, when you dissolve them in water, hydroxide ions are formed. So, where do these hydroxide ions come from? In a solution of a weak base, the 1- ions come from the water! The formula of this weak base does NT contain hydroxide ( 1- ). N3(g) + (l) <===> N4 1+ (aq) + 1- (aq) Whenever you write an equation showing the reaction of a weak base with water, you should always include the water in the equation (as is done in the equation above). If you write the equation without including the water, it will not be balanced: Wrong -----> Right -----> N3(g) <===> N4 1+ (aq) + 1- (aq) N3(g) + (l) <===> N4 1+ (aq) + 1- (aq) , A.J. Girondi

14 Problem 7. Write balanced equations showing the behavior of the following weak bases in water. Keep in mind that the particle of weak base will accept a 1+ ion from water. a. Br 1- (aq) Br 1- (aq) + (l) <===> b. S 2- (aq) S 2- (aq) + (l) <===> Problem 8. Write balanced equations which illustrate the behavior of the substances listed below when they are added to water. Refer back to the examples in previous sections of this chapter. Be sure to include charges on ions. a. Br (a strong acid) b. Li (a strong base) c. N2 (a weak acid) d. CN 1- (a weak base) (Check to be sure that you showed charges on any ions in the products.) There are two reactions involved in an equilibrium system. In the acetic acid system, note that in the forward reaction (--->) we can classify one substance as an acid and one as a base, and we can do the same thing for the reverse reaction (<---): C (l) <====> C (aq) (aq) acid base base acid We are making use of the Bronsted Lowry definitions of acid and base here. What molecule is the proton donor in the forward (--->) reaction? {12} proton donor in the reverse reaction? {13} What ion is the Problem 9. In the three equations below, label each substance as an acid or a base: a. F + 2 <===> F 1- b. C Br <===> 2C3 + Br 1- c. CN + N3 <===> CN 1- + N4 1+ At equilibrium, the system contains mostly reactants. This was also the case for the weak acids. In other words, the weak bases do not form a high concentration of 1- ions. Even though weak bases do not contain 1-, it is still the 1- ion that makes the solution basic. Notice that the 1- ions which are produced in a solution of a weak base are formed indirectly. By that we mean that the 1- ions do not come from the base itself, but they come from the water! Examine the following equation , A.J. Girondi

15 In solutions of weak bases, the 1- ions come from the water! N2 1- (aq) + (l) <===> N2(aq) + 1- (aq) base acid acid base As you can see, some negative ions (anions) like N2 1- can acts as weak bases in water: In solutions of strong bases like Na, the 1- ion comes from the base itself. Water is not shown in the equation which represents a strong base in water: Na ----> Na Explain the difference in the source of the 1- ions in solutions of strong bases versus solutions of weak bases: {14} Since weak bases involve equilibrium systems, an equilibrium expression can be written. For the system below: N2 1- (aq) + (l) <===> N2(aq) + 1- (aq) The equilibrium expression = [N 2 ] [ 1- ] [N 2 1 ] Water is omitted since it is a pure liquid. This expression is equal to a constant which is given the symbol Kb. It is known as the dissociation constant of a weak base: K b = [N 2 ] [1- ] [N 2 1 ] Notice that Kb gets larger as the concentration of 1- increases. You probably already realize that the larger the Kb value, the stronger the base. A very common and important weak base is ammonia, N3. Ammonia is very useful in the production of explosives and fertilizers. When it is dissolved in water, the solution is usually called "ammonium hydroxide." When you look at the reaction, you can guess why: ammonia water ammonium ion hydroxide ion N3(g) + (l) <===> N4 1+ (aq) + 1- (aq) base acid acid base The electron dot structure of ammonia is shown in the space at the right. What feature makes it a base (even though it's a weak one)? {15} N , A.J. Girondi

16 The products of the reaction are an ammonium ion and a hydroxide ion. owever, since this is a weak base, a better name for the solution would be "ammonia water." Explain why: {16} In fact, if you put 100 ammonia molecules in water, only one would be dissociated at equilibrium. Thus, for every 100 ammonia molecules put into a solution, only one 1- ion would be formed at equilibrium. When 100 of these are added to water N3(g) + (l) <===> N4 1+ (aq) + 1- (aq) 99 1 Number of particles present at equilibrium Problem equation. 10. The equations below involve weak bases in water. Write the Kb expression for each a. CN 1- (aq) + 2(l) <===> CN(aq) + 1- (aq) Kb = b. N3(aq) + 2(l) <===> N4 1+ (aq) + 1- (aq) Kb = Three weak bases are listed below along with their Kb values. Which is strongest? {17} Weakest? {18} Base Dissociation Constant ammonia, N3 Kb = 1.8 X 10-5 fluoride ion, F 1- Kb = 1.4 X sulfite ion, S3 2- Kb = 1.8 X 10-7 It is not easy to identify a weak base just by looking at its formula. You would have to draw its electron-dot structure to see if it contains an unshared pair of valence electrons, and you would have to study how it reacts with water. Some particles like the chloride ion, Cl 1-, do not act as bases even though they do have unshared pairs of valence electrons. ne simple way of explaining this is to say that if the Cl 1- ion were to accept a proton, it would form a molecule of Cl. owever, Cl is a strong acid, and strong acids are completely dissociated in water solutions. Therefore, Cl cannot form. Particles such as Cl 1- which would form a molecule of a strong acid by accepting a proton cannot, therefore, serve as bases. They are neutral particles. The anions (negative ions) which are found in the six strong acids are neutral anions. As mentioned earlier in this chapter, they include Cl 1-, Br 1-, I 1-, S4 2-, N3 1-, and Cl4 1-. Keep in mind that the reactions we are considering here all occur in water. Therefore,let's say that the Cl 1- ion is trying to accept a proton from water: Cl 1- (aq) + 2(l) ----> Cl(aq) + 1- (l) <----- This reaction WILL NT happen! , A.J. Girondi

17 Why won't the reaction shown in the equation above happen? Well, let's just say that the reaction would result in the formation of a strong acid (Cl in this example). Strong acid molecules like Cl do not exist in water because they are completely dissociated. Therefore, the reaction does not occur. ACTIVITY 21.7 Comparing the Conductivity of Strong and Weak Bases This activity is like Activity 21.4 in which you determined the relative strengths of a few acids. The method involves measuring the conductivity of a solution. When ions are present in a solution, it is possible for the solution to conduct electricity. ther things being equal, the greater the concentration of ions in a solution, the greater the electrical current that will pass through the solution. The number of ions which form when an acid or base is added to water depends on the degree of dissociation. Since stronger acids and bases dissociate much more than weaker ones do, they are much better conductors of electric current. The greater the electrical conductivity of an acid or base solution, the stronger it is. Because of your experience in Activity 21.4, you should already know how to use the conductivity device. Use it to measure the conductivity of the two bases listed in Table Record your observations in the spaces provided in the table. If the apparatus has a meter, record the meter reading in the table. If it has a light bulb, record the strengths of the acids as high, medium, or low depending on the brightness of the bulb. If the bulb does not light, this is probably because the ion concentration is too low to allow enough current to flow to light the bulb. It does not necessarily mean that there are no ions in the solution. (int: one of the acids should be rated "high;" one should be rated "low." You determine which is which. Table 21.3 Conductivity of Basic Solutions Base Formula Base Name Conductivity 0.1M Na sodium hydroxide 0.1M N3 ammonia Based on the results, what conclusions can you draw about the relative strengths of these two bases? {19} Even though both base solutions are 0.1M, they do not conduct the same amount of electric current and are not equally strong. Why not? {20} SECTIN 21.8 Polyprotic Acids There are only six acids which are generally recognized as being strong (100% dissociated). They include Cl, N3, 2S4, Br, I, and Cl4. (You should have memorized these by now!) The number of hydrogens in the formula has nothing to do with the acid's strength. Thus, 2S4 is not twice as strong as the other strong acids. Phosphoric acid, 3P4, is weak even though it contains three hydrogens! Remember, it is the percentage of dissociation that determines an acid's strength. (In other words, how well the acid molecule reacts with water.) That is, the strength is determined by how many 1+ ions actually form when you put the acid particle in water, not by how many 's are in the acid's formula. If , A.J. Girondi

18 an acid can provide only one 1+ ion, it is said to be monoprotic. The general term for acids which can yield more than one 1+ ion is "polyprotic." If an acid can provide two 1+ ions, it is "diprotic." completely as shown in the equation below: Sulfuric acid is strong because it ionizes 2S4(aq) + 2(l) ----> 3 1+ (aq) + S4 1- (aq) Notice that only one hydrogen formed, even though 2S4 contains two hydrogens. The second hydrogen is now part of the S4 1- ion, and it is very hard for a second 1+ to break away since the S4 1- ion, with its negative charge, strongly "holds on" to it. As shown in the equation below, a few do manage to break away, and as a result, the second hydrogen makes a small, very insignificant contribution to the total number of 1+ ions in a solution of sulfuric acid. The S4 1- ion is a relatively weak acid: S4 1- (aq) + 2(l) <===> 3 1+ (aq) + S4 2- (aq) This equilibrium reaction does not produce much Therefore, when describing the behavior of sulfuric acid in water, it is sufficient for our purposes to say that it undergoes only one significant step: 2S4(aq) + 2(l) ----> 3 1+ (aq) + S4 1- (aq) Now, what we have just discussed is the behavior of sulfuric acid in pure water. owever, sulfuric acid is diprotic and it can give up two hydrogen ions when it reacts with a strong base instead of with water. You will learn more about this when you study acid-base reactions in the next chapter. Not all acids with more than one "" are polyprotic. As mentioned earlier in this chapter, acetic acid has the formula C232. nly the first hydrogen in the formula can dissociate - at all - to form 1+. The other three hydrogens are part of the acetate ion (a polyatomic ion), C232 1-, and they are bonded differently in the molecule. As a result, they cannot form 1+ ions. Therefore, it is only a monoprotic acid. Sometimes, those who work primarily with carbon compounds (organic chemists) write the formulas for acids differently. For example, instead of writing acetic acid as C232, they may write C3C. The hydrogen that forms 1+ now appears at the end of the formula instead of the beginning. This alternate way of writing formulas gives more of a clue as to the structure of the molecule. Note the structure of acetic acid shown at right. C C Acetic Acid C3C or C232 In the acetic acid molecule, it is the hydrogen which is bonded to oxygen which can dissociate in water as an 1+ ion. The bond is polar, and so is the water molecule. Thus there is an attraction between them, and water can act as a base and "pull" a few of those particular 's off of the acetic acid molecule. The C bonds are not polar, so those 's are not pulled away by water at all. Acetic acid has only a 1-step dissociation. The following equation showing electron-dot structures may be helpful: , A.J. Girondi

19 C C nonpolar bond polar bond polar water molecule Acetic Acid Water ydronium Ion Acetate ion C C 1 ne final word about the hydronium ion. As you know, the hydronium ion forms when water (which has 2 unshared pairs of electrons) acts as a base and accepts a proton. This is happening in the equation shown above. Since the hydronium ion, 3 1+, ion still contains one unshared pair of electrons, students often ask why 3 1+ cannot accept a second proton to form an ion with the formula The proposed equation for this reaction is shown below This reaction does NT occur. Suggest a reason why the hydronium ion cannot act as a base by accepting a proton. {21} , A.J. Girondi

20 SECTIN 21.9 LEARNING UTCMES This is the end of Chapter 21. Check the learning outcomes below and arrange to take the exam on Chapter 21. Then, go on to Chapter 22 which is a continuation of the subject of acids and bases. 1. List the general properties of acids and bases. 2. Compare and contrast the definitions of acids and bases according to the Arrhenius and the Bronsted Lowry models. 3. Write balanced equations showing the dissociation of strong and weak acids and bases in water. 4. Given a list of acids, classify them as strong or weak. 5. Given a list of bases, classify them as strong or weak. 6. Explain why some acids are strong, while others are weak. 7. Explain why some bases are strong, while others are weak , A.J. Girondi

21 SECTIN Answers to Questions and Problems Questions: {1} hydroxide; {2} Bronsted-Lowry; {3} weak; {4} better; {5} 2; {6} F 1- ; {7} strongest is Cl and weakest is acetic; {8} They do not all dissociate into ions to the same extent. Cl dissociates most, while acetic dissociates least; {9} phosphoric; {10} hypochlorous; {11} An unshared pair of electrons on the fluoride ion; {12} C232; {13} 3 1+ ; {14} In solutions of strong bases, the 1- comes from the base itself, whereas, in solutions of weak bases, the 1- comes from the water that the weak base reacts with; {15} An unshared pair of electrons on the nitrogen atom; {16} The reverse reaction is better than the forward reaction, so there is more ammonia and water present at equilibrium; {17} N3; {18} F 1- ; {19} Sodium hydroxide is strong but ammonia is weak; {20} The 0.1M Na dissociates to a high degree to form a lot of 1- ions, but 0.1M ammonia does not; {21} The positively-charged 3 1+ ion will repel the positively-charged 1+ ion too much; Problems: 1. a. Br(aq) ----> 1+ (aq) + Br 1- (aq) b. I(aq) ----> 1+ (aq) + I 1- (aq) c. Cl4(aq) ----> 1+ (aq) + Cl4 1- (aq) 2. a. K(s) ----> K 1+ (aq) + 1- (aq) b. Li(s) ----> Li 1+ (aq) + 1- (aq) c. Cs(s) ----> Cs 1+ (aq) + 1- (aq) 3. a. Br(aq) ----> 1+ (aq) + Br 1- (aq) Br(aq) + 2(l) ----> 3 1+ (aq) + Br 1- (aq) b. Cl4(aq) ----> 1+ (aq) + Cl4 1- (aq) Cl4(aq) + 2(l) ----> 3 1+ (aq) + Cl4 1- (aq) 4. a. Cl3(aq) + 2(l) <===> 3 1+ (aq) + Cl3 1- (aq) b. Cl(aq) + 2(l) <===> 3 1+ (aq) + Cl 1- (aq) 5. a. K a = [ 3 1+ ] [N 2 1 ] [N 2 ] b. K a = [ 3 1+ ] [CN 1 ] [CN] 6. a. Rb(s) ----> Rb 1+ (aq) + 1- (aq) b. Ba()2(s) ----> Ba 2+ (aq) (aq) 7. a. Br 1- (aq) + (l) <===> Br(aq) + 1- (aq) b. S 2- (aq) + (l) <===> S (aq) 8. a. Br(aq) + 2(l) ----> 3 1+ (aq) + Br 1- (aq) b. Li(s) ----> Li 1+ (aq) + 1- (aq) c. N2(aq) + (l) <===> N2 1- (aq) (aq) d. CN 1- (aq) + (l) <===> CN(aq) + 1- (aq 9. a. acid, base, acid, base b. base, acid, acid, base c. acid, base, base, acid 10. a. K b = [CN] [1- ] [CN 1- ] b. K b = [N 4 1+ ] [ 1- ] [N 3 ] , A.J. Girondi

22 SECTIN Student Notes , A.J. Girondi